1. Field of the Invention
The present invention relates to a boundary acoustic wave element and a boundary acoustic wave device which are used in resonators and band-pass filters, for example, and manufacturing methods for the same. More specifically, the present invention relates to a boundary acoustic wave element and a boundary acoustic wave device which are provided with a heat dissipation structure, and manufacturing methods for the same.
2. Description of the Related Art
Recently, surface acoustic wave filter devices are widely used as band-pass filters in the RF stage of mobile telephones. High electric power resistance is significantly demanded for this type of band-pass filters.
Accordingly, Japanese Unexamined Patent Application Publication No. 2006-14096 proposes a structure which quickly releases heat generated when large electric power is applied to an IDT electrode, thereby suppressing migration of the IDT electrode and enhancing electric power resistance.
In a surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2006-14096, a surface acoustic wave element is mounted on the upper surface of a circuit board, and the periphery of the surface acoustic wave element is sealed with molding resin. In the above-mentioned surface acoustic wave element, a functional electrode portion including the IDT electrode is formed on one principal surface of a piezoelectric substrate. A grounding annular electrode is formed so as to surround this functional electrode portion.
The above-mentioned surface acoustic wave element is mounted on the upper surface of the circuit board in such a way that its one principal surface side where the above-mentioned functional electrode portion and the grounding annular electrode are formed is opposed to the upper surface of the circuit board. The grounding annular electrode is electrically connected to a through-conductor provided so as to extend through the circuit board from the upper surface to the lower surface, and the through-conductor is electrically connected to a grounding conductor provided on the lower surface of the circuit board.
In this structure, the grounding annular electrode is electrically connected to a portion of the functional electrode portion including the IDT electrode which is connected to the ground potential. Therefore, heat generated in the IDT electrode is readily released to the exterior through the grounding annular electrode, the through-conductor, and the grounding conductor. Thus, an adverse influence due to heat is suppressed, making it possible to enhance electric power resistance.
Recently, on the other hand, boundary acoustic wave devices are attracting attention instead of surface acoustic wave devices. In surface acoustic wave devices, a space for vibrations of a portion where the IDT electrode is arranged must be provided within the package. In contrast, such a space can be omitted in the boundary acoustic wave devices. Thus, the boundary acoustic wave devices are small-sized.
FIG. 11 is a schematic front sectional view showing an example of a boundary acoustic wave device according to the related art.
A boundary acoustic wave element 501 has a piezoelectric substance 502. An IDT electrode 504, and reflectors 505 and 506 are formed on the upper surface of the piezoelectric substance 502. A dielectric substance 503 is laminated on the upper surface of the piezoelectric substance 502. That is, a functional electrode portion including the IDT electrode 504 is disposed at the interface between the piezoelectric substance 502 and the dielectric substance 503. A sound-absorbing layer 507 is provided on the dielectric substance 503. The sound-absorbing layer 507 is provided to absorb bulk waves that have propagated to the upper surface side of the dielectric substance 503, and suppresses unwanted spurious response.
In the case of the boundary acoustic wave element 501 as well, heat is generated in the functional electrode portion including the IDT electrode 504 when in actual use. Thus, there is a fear that migration of the IDT electrode or the like may occur due to this heat, causing electric power resistance to be decreased.
Therefore, it is considered desirable that the boundary acoustic wave element 501 also be provided with a heat dissipation structure such as one described in Japanese Unexamined Patent Application Publication No. 2006-14096.
However, in the case where the heat dissipation structure described in Japanese Unexamined Patent Application Publication No. 2006-14096 is adopted for the boundary acoustic wave element 1, the boundary acoustic wave element becomes large. Thus, the advantage of the boundary acoustic wave element, namely that it allows miniaturization, is greatly impaired. That is, in the heat dissipation structure described in Japanese Unexamined Patent Application Publication No. 2006-14096, the grounding annular electrode is arranged to surround the functional electrode portion including the IDT electrode. Accordingly, to provide a grounding annular electrode in the boundary acoustic wave element 501, naturally, on the upper surface of the piezoelectric substance 2, the grounding annular electrode must be formed in the peripheral region including the functional electrode portion such as the IDT electrode 504. Thus, the area of the above-mentioned boundary interface becomes large, that is, the plan area of the boundary acoustic wave element 501 becomes large, leading to an inevitable increase in mounting space.
In a case where the above-mentioned grounding annular electrode is provided in the boundary acoustic wave element 501, it is necessary to provide a connecting conductor for leading out the grounding annular electrode to the lower surface of the piezoelectric substance 502, or to the outer side surfaces of the dielectric substance 503 and sound-absorbing layer 507, which adds complexity to the structure of the boundary acoustic wave element 501 and also leads to high manufacturing cost.